Planning and Execution of a Large Mass Concrete Placement with Different Insulation Levels

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Title: Planning and Execution of a Large Mass Concrete Placement with Different Insulation Levels

Author(s): Boris Haranki, Harini Santhanam, and Ufuk Dilek

Publication: Special Publication

Volume: 325

Issue:

Appears on pages(s): 8.1-8.14

Keywords: mass concrete; thermal control plan; fly ash; thermal cracking; heat of hydration; curing; temperature; construction, insulation

Date: 7/25/2018

Abstract:

This paper summarizes the planning and execution stages for a large mass concrete placement. The subject structure was a component of an industrial facility, consisting of a large mat foundation on grade. The planning and execution of this critical mass placement consisted of multiple phases:

For the first phase of the work, a laboratory study was performed for the purpose of developing a concrete mixture that will perform satisfactorily meeting the mass concrete objectives. The laboratory development phase consisted of conventional strength based design of a mix meeting additional specification requirements for control of heat of hydration. The project specifications required the use of 35% ash and imposed a cap on total cementitious materials content. The selected proportions were then batched and placed in a mock-up consisting of a 3 ft (91 cm) by 3 ft (91 cm) by 3 ft (91 cm) cube for the purposes of observing peak temperatures exhibited by the mixture and the temperature differentials. This member size was selected for the mock-up as it is typically the delineating minimum member dimension for mass concrete.

As part of the next phase of the preparatory work, a thermal simulation of the actual placement was performed using public domain software. Based on a review of the findings from these efforts, the specifics of the thermal control plan for the actual placement were finalized. As part of the thermal control plan analysis, target placement temperatures were recommended to control maximum temperatures that prevent occurrence of Delayed Ettringite Formation (DEF), in light of the heat rise of the mix.

The actual placement of nearly 760 m3 (1000 CY) was performed in early fall weather conditions over 9 hours. Concrete was chilled to meet the delivery temperatures and insulated per the thermal control plan specifics. The placement temperature was accomplished by starting the placement at night and with the use of chilled water, ice, and liquid nitrogen (as needed) to lower the placement temperature during the day. Upon completion, the placement was insulated using three different insulation regimes. The resulting concrete temperatures were monitored and enabled observation of differences between each insulation regime. This phase also served as a confirmatory phase of the specific insulation attributes indicated by the thermal analysis in the previous phase. The placement was completed successfully with internal temperatures and gradients controlled within the desired ranges.

The specific selections in mixture proportions and the delivery temperature requirements protected the concrete against high internal temperatures and potential of DEF, while the insulation regimen protected the concrete against rapid cooling of the surface and occurrence of thermal gradients between core and perimeter. The multiple insulating regimens implemented during actual placement were instrumental in confirming the effects of insulation on peak temperature and loss of heat, as indicated by the analytical simulation.